Numerous spinning assemblies have been devised for the production of hollow fibers, including multicomponent fibers of the sheath/core type. Particularly, devices have been proposed for ensuring uniform supply of homogenous fiber-forming liquid and sheath liquid to the orifices of a spinnerette with the object of producing hollow fibers identical in denier and other characteristics. These spinnerettes use a means for supplying the bore fluid positioned in the spinning orifice for forming the hollow fiber. Usually, a tube is used for this purpose and a fluid is injected from the tube into the bore of the fiber being formed to maintain the shape of the fiber until the polymeric materials making up the fiber are coagulated or solidified.
A typical spinning assembly such as described in U.K. Patent No. 830,441 for a multi-component hollow fiber comprises a front and back plate spaced apart from but facing each other so as to provide a liquid channel there between. The front plate is provided with an extrusion orifice therethrough, and at least one of the plates, on its side facing the other plate, is provided with a plateau-type protrusion so as to constrict the liquid channel in a region surrounding the extrusion orifice entrance and, thus, cause streams of the sheath-forming material to converge substantially radially towards the orifice entrance. A tube is positioned in the orifice entrance to supply the bore fluid. However, a continuing problem is the uniform supply of sheath-forming material to the multicomponent hollow fiber membrane.
Most spinnerettes of this type are made largely by hand, one at a time. As a result, parts made for one spinnerette will not always fit another spinnerette. When parts are not interchangeable, any damage to one part of the spinnerette assembly may render the entire assembly useless.
In assembling or cleaning conventional hollow-fiber spinnerettes, it is very easy to slightly bend the fluid-injection tube, such that it is off center of the spinning orifice. When this happens, the spinnerette cannot be used again.
Another related problem of conventional spinnerettes for multicomponent hollow fibers is that the sheath and core of the hollow fiber are not concentric. Concentricity of the sheath and core are important to obtain uniform fibers. Concentricity of the sheath and core was customarily obtained by adjustment of metering surfaces to regulate (meter) the flow of the polymers. The metering surfaces are produced by closely machining two surfaces so as to produce a narrow opening which will effectively meter polymer at a uniform pressure and rate as it is being extruded. For example, a plateau-type spinnerette is known in the prior art. In addition, U.S. Pat. No. 3,458,615 discloses a method for the circumferential metering of the polymer sheath to be extruded about the polymeric core involving the creation of an annular wedged-shaped flow of sheath polymer around the axially-contained fluid stream.
In order to maintain the concentricity of the sheath and core, other spinnerettes have been provided with members for centering the tube and the bore of the spinnerette plate. For example, U.S. Pat. No. 4,493,629 describes a modular spinnerette assembly fitted with multiple screws threaded through the spinnerette plate to center the tube and orifice of the spinnerette. These adjusting screws are unreliable and are prone to error when the spinnerette is disassembled, cleaned and then reassembled.
Many devices have been proposed for ensuring uniform supply of homogenous sheath-forming liquid to the orifices of a multi-orifice spinnerette with the object of producing multiple hollow fibers with concentric layers, identical in denier and other characteristics. Such devices usually involve variations in the diameter or the location of orifices and single or multiple spinnerette plates. Lack of concentricity and uniformity remain a problem in the manufacture of such multicomponent fibers.